NEMA Design B Motors

The NEMA design B motor is the basic integral horsepower motor. It is a three-phase motor designed with normal torque and normal starting current and generally has a slip at the rated load of less than 4%. Thus, the motor speed in revolutions per minute is 96% or more of the synchronous speed for the motor. For example, a four-pole motor operating on a 60-Hz line frequency has a synchronous speed of 1800 rpm or a full-load speed of
1800 – (1800 x slip) – 1800 – (1800 x 0.04) = 1800 – 72 = 1728 rpm
or
1800 x 0.96= 1728 rpm
In general, most three-phase motors in the 1- to 200-hp range have a slip at the rated load of approximately 3% or, in the case of four-pole motors, a full-load speed of 1745 rpm. Figure 1.1 shows the typical construction for a totally enclosed, fan-cooled NEMA design B motor with a die-cast aluminum single-cage rotor.
Figure 1.2 shows the typical speed-torque curve for the NEMA design B motor. This type of motor has moderate starting torque, a pull-up torque exceeding the full-load torque, and a breakdown torque (or maximum torque) several times the full-load torque. Thus, it can provide starting and smooth acceleration for most loads and, in addition, can sustain temporary peak loads without stalling. The NEMA performance standards for design B motors are shown in Tables 1.1-1.3.FIGURE 1.1 NEMA design B totally enclosed, fan-cooled polypha induction motor. (Courtesy Magnetek, St. Louis, MO.)
In the past, there were no established standards for efficiency or power factor for NEMA design B induction motors. However, NEMA had established standards for testing and labeling induction motors. Recently, NEMA has established efficiency standards for energy-efficient polyphase induction motors. These standards are discussed in detail in topic 2.1.1.2

NEMA Design A Motors

The NEMA design A motor is a polyphase, squirrel-cage induction motor designed with torques and locked-rotor current that exceed the corresponding values for NEMA design B motors. The criterion for classification as a design A motor is that the value of the locked-rotor current be in excess of the value for NEMA design B motors. The NEMA design A motor is usually applied to special applications that cannot be served by NEMA design B motors, and most often these applications require motors with higher than normal breakdown torques to meet the requirements of high transient or short-duration loads. The NEMA design A motor is also applied to loads requiring extremely low slip, on the order of 1% or less.FIGURE 1.2 NEMA design B motor speed-torque curve.1.1.3

NEMA Design C Motors

The NEMA design C motors is a squirrel-cage induction motor that develops high locked-rotor torques for hard-to-start applications. Figure 1.3 shows the construction of a drip-proof NEMA design C motor with a double-cage, die-cast aluminum rotor. Figure 1.4 shows the typical speed torque curve for the NEMA design C motor. These motors have a slip at the rated load of less than 5%.

NEMA Design D Motors

The NEMA design D motor combines high locked-rotor torque with high full-load slip. Two standard designs are generally offered, one
TABLE 1.2 Breakdown Torque of NEMA Design A and B Motors8-15

hp

Synchronous

speed, 60 Hz

3600 rpm

1800 rpm

1200 rpm

900 rpm

1

—

300

265

215

1.5

250

280

250

210

2

2 10

270

240

210

:.i

230

250

230

205

5

215

225

215

205

7,5

200

215

205

200

10

200

200

200

200

15

200

200

200

200

20

200

200

200

200

25

200

200

200

200

30

200

200

200

200

40

200

200

200

200

50

200

200

200

200

00

200

200

200

200

75

200

200

200

200

100

200

200

200

200

125

200

200

200

200

150

200

200

200

200

200

200

200

200

200

250

175

175

175

175

a Single-speed, polyphase, squirrel-cage, medium-horsepower motors with continuous ratings (percent of full-load torque). b For other speeds and ratings, see NEMA Standard MG1-12.39.1. Source: Reprinted by permission from NEMA Standards Publication No. MG1-1987 Motors and Generators, copyright 1987 by the National Electrical Manufacturers Association.
with full-load slip of 5-8 % and the other with full-load slip of 813%. The locked-rotor torque for both types is generally 275-300% of full-load torque; however, for special applications, the locked-rotor torque can be higher. Figure 1.5 shows the typical speed-torque curves for NEMA design D motors. These motors are recommended for cyclical loads such as those found in punch presses, which have

TABLE 1.3 Locked-Rotor Current of NEMA Design B, C, and D

Motorsa,b,c

Locked-rotor

NEMA design

hp

current A

letter

Code letter

1

30

B, D

N

1.5

■10

B, D

M

2

50

B, D

L

3

64

B, C, D

K

5

92

B, C, D

•I

7.5

127

B, C, D

11

10

162

B, C, D

II

].->

232

B, C, D

G

20

290

B, C, D

c,

365

B, C, D

G

;:o

435

B, C, D

G

10

580

B, c, n

G

50

725

B, c, n

G

60

870

B, C, D

G

75

1085

B, C, D

G

100

1450

B, C, D

G

125

1815

B, C, D

G

150

2170

B, C, D

G

2i)0

2900

B, C

G

250

3650

B

G

a Three-phase, 60-Hz, medium-horsepower, squirrel-cage induction motors rated at 230 V.
b For other horsepower ratings, see NEMA Standard MG1-12.35.
c The locked-rotor current for motors designed for voltages other than 230
V shall be inversely proportional to the voltage.
Source: Reprinted by permission from NEMA Standards Publication No. MG1-1987, Motors and Generators, copyright 1987 by the National Electrical Manufacturers Association.FIGURE 1.3 NEMA design C drip-proof polyphase induction motor. (Courtesy Magnetek, St. Louis, MO.)
stored energy systems in the form of flywheels to average the motor load and are excellent for loads of short duration with frequent starts and stops. The proper application of this type of motor requires detailed information about the system inertia, duty cycle, and operating load as well as the motor characteristics. With this information, the motors are selected and applied on the basis of their thermal capacity.1.1.5

Wound-Rotor Induction Motors

The wound-rotor induction motor is an induction motor in which the secondary (or rotating) winding is an insulated polyphase winding similar to the stator winding. The rotor winding generally terminates at collector rings on the rotor, and stationary brushes are in contact with each collector ring to provide access to the rotor circuit. A number of systems are available to control the secondary resistance of the motor and hence the motor’s characterstics. The use and application of wound-rotor induction motors have been limited mostly to hoist and crane applications and special speed-controlFIGURE 1.4 NEMA design C motor speed-torque curve.
applications. Typical wound-rotor motor speed-torque curves for various values of resistance inserted in the rotor circuit are shown in Fig. 1.6. As the value of resistance is increased, the characteristic of the speed-torque curve progresses from curve 1 with no external resistance to curve 4 with high external resistance. With appropriate control equipment, the characteristics of the motor can be changed

a Single-speed, polyphase, squirrel-cage, medium-horsepower motors with continuous ratings (percent of full-load torque), MG1-12.39.2. Source: Reprinted by permission from NEMA Standards Publication No. MG1-1987, Motors and Generators, copyright 1987 by the National Electrical Manufacturers Association.FIGURE 1.5 NEMA design D motor speed-torque curves: 5-8% and 8-13% slip.
by changing this value of external rotor resistance. Solid-state inverter systems have been developed that, when connected in the rotor circuit instead of resistors, return the slip loss of the motor to the power line. This system substantially improves the efficiency of the wound-rotor motor used in variable-speed applications.FIGURE 1.6 Wound-rotor motor speed-torque curves: 1, rotor short-circuited; 2-4, increasing values of external resistance.1.1.6

Multispeed Motors

Motors that operate at more than one speed, with characteristics similar to those of the NEMA-type single-speed motors, are also available. The multispeed induction motors usually have one or two primary windings. In one-winding motors, the ratio of the two speeds must be 2 to 1; for example, possible speed combinations are 3600/
1800, 1800/900, and 1200/600 rpm. In two-winding motors, the ratio of the speeds can be any combination within certain design limits, depending on the number of winding slots in the stator. The most popular combinations are 1800/1200, 1800/900, and 1800/ 600 rpm. In addition, two-winding motors can be wound to provide two speeds on each winding; this makes it possible for the motor toFIGURE 1.7 Speed-torque curves for a variable-torque, one-winding, two-speed motor.
operate at four speeds, for example, 3600/1800 rpm on one winding and 1200/600 rpm on the other winding.
Multispeed motors are available with the following torque characteristics.
Variable Torque. The variable-torque multispeed motor has a torque output that varies directly with the speed, and hence theFIGURE 1.8 Speed-torque curves for a multispeed variable-torque motor with two windings, two speeds, and a four-pole to six-pole ratio.
horsepower output varies with the square of the speed. This motor is commonly used with fans, blowers, and centrifugal pumps to control the output of the driven device. Figure 1.7 shows typical speed-torque curves for this type of motor. Superimposed on the motor speed-torque curve is the speed-torque curve for a typical fan where the input horsepower to the fan varies as the cube of the fan speed. Another popular drive for fans is a two-windingFIGURE 1.9 Speed-torque curves for a constant-torque, one-winding, two-speed motor.
two-speed motor, such as 1800 rpm at high speed and 1200 rpm at low speed. Figure 1.8 shows the typical motor speed-torque curve for the two-winding variable-torque motor with a fan speed-torque curve superimposed.
Constant Torque. The constant-torque multispeed motor has a torque output that is the same at all speeds, and hence the horsepowerFIGURE 1.10 Speed-torque curves for a constant-horsepower, one-winding two-speed motor.
output varies directly with the speed. This motor can be used with friction-type loads such as those found on conveyors to control the conveyor speed. Figure 1.9 shows typical speed-torque curves.
Constant Horsepower. The constant-horsepower multispeed motor has the same horsepower output at all speeds. This type of motor is used for machine tool applications that require higher torques at lower speeds. Figure 1.10 shows typical speed-torque curves.